The present invention relates to a discharge protection circuit for battery-operated devices such as portable electric lights, electric tools, and the like. More particularly, the invention relates to a discharge protection circuit of relatively simple design that protects re-chargeable batteries from damage caused by overdischarging.
Rechargeable batteries are used to power a wide variety of battery-operated or "cordless" electric devices. Ideally, a rechargeable battery could be charged and discharged an infinite number of times. In practice, batteries have a limited useful life, which can be further shortened if the batteries are damaged. Overdischarging is a significant cause of damage to rechargeable batteries. Typically, overdischarging occurs as a result of the electric device being inadvertently left on. When this happens, the battery (or batteries) in the device continues to discharge until the voltage of the battery drops to a zero or near zero value. Allowing the voltage to drop to such a low value typically causes cell reversal and damage to the battery. Often, the damage is so severe that the battery must be replaced. At a minimum, the battery's ability to fully recharge is affected.
Devices that prevent overdischarge are available. Many available devices attempt to solve the problem of overdischarge by monitoring the voltage level of the battery and disconnecting a battery from its load once the battery has discharged to a predetermined level. One difficulty with this type of design is that a discharged battery's open circuit voltage returns to a level that is nearly the same as a fully charged battery ("open circuit recovery"). Thus, in cordless devices equipped with voltage-monitoring protection devices, a battery will continue to discharge after open circuit recovery occurs, further depleting the battery. Using voltage hysteresis in a voltage monitoring circuit does not solve this problem.
Some protection devices provide a latching function to prevent discharge of an already depleted battery. However, most methods and components used to provide a latching function are complex and costly. Expensive protection devices are, in general, not suitable for cordless devices that sell at relatively low prices. Yet, if inexpensive and simple protection devices were available, protection devices could be fitted in many inexpensive cordless devices and significant cost savings could be realized for nearly all cordless devices.